MgO single crystals are applied to a wide variety of uses, such as a substrate for an oxide superconductor thin film, a substrate for oxide dielectric thin film, a high heat-conductive substrate, an optical lens, a window material for infrared ray transmission, and a target material used in vapor deposition or sputtering for producing, e.g., a protective film for plasma display panel (PDP). Particularly, the MgO single crystal has excellent lattice match with an oxide superconductor and a coefficient of thermal expansion equivalent to that of the oxide superconductor, and further has a low permittivity, and hence recently has drawn attention as a substrate for oxide superconductor thin film used in a high frequency device.
MgO has a high vapor pressure, and therefore a MgO single crystal is generally produced by an arc electrofusing method. The arc electrofusing method is a method in which electrodes are inserted into magnesia clinker as a raw material in order to melt the raw material, and a skull layer is formed from the melted raw material and the raw material melt is kept by self-lining to effect crystallization. Therefore, the arc electrofusing method has a problem in that control of the conditions for forming a single crystal is difficult, thus making it difficult to obtain a single crystal having a large size.
For solving the problem, there have been proposed a method for producing a MgO single crystal having a large size, in which powdered magnesia is put on the raw material magnesia clinker layer to stabilize the closure and temperature in the electric furnace (patent document 1), and a method for producing a MgO single crystal having a large size, in which the furnace is densely filled with a high-purity raw material having a magnesia with purity of 99.8% or more (patent document 2).
The above improvements of the arc electrofusing method make it possible to produce a MgO single crystal having a relatively large size. However, the arc electrofusing method is not a method in which a single crystal is allowed to grow successively on a seed crystal, such as a conventional single crystal growth method, e.g., a pulling method, and therefore the arc electrofusing method has a fundamental problem of that it is difficult to obtain a single crystal having excellent crystalline properties and a large size.
For solving the problem, the improvement of the arc electrofusing method has been proposed in which the cooling rate is controlled by changing the electrode pulling speed to restrict the number of subboundaries and the dislocation density (patent document 3), and it has been reported that a MgO single crystal substrate having excellent crystalline properties can be obtained by this method.
A MgO single crystal substrate having a large size and excellent crystalline properties can be obtained by the above method; however, when, for example, an oxide superconductor thin film is formed on the resultant substrate, a problem arises in that the oxide superconductor thin film obtained is not uniform in the superconducting properties and is not stable in the performance. The reason for this is presumed that MgO is easily reacted with moisture or carbonic acid gas in air to change the surface state of the substrate with time.
As a method for improving the surface of the MgO single crystal substrate, a method in which the substrate surface is treated with weakly acidic cleaning water having a specific pH (patent document 4), a method in which the substrate surface is subjected to heat treatment after the removal step by polishing (patent document 5), and a method in which the calcium (Ca) and silicon (Si) contents are specified to improve the surface smoothness of the MgO single crystal substrate (patent document 6) have been proposed.
By the above methods for improving the surface of the MgO single crystal substrate, a superconductor thin film formed on the substrate can be improved in superconducting properties to some extent, but satisfactory results are not obtained. Therefore, a MgO single crystal substrate advantageous in that, particularly, a superconductor thin film formed on the substrate exhibits excellent superconducting properties is desired.
[Patent document 1] Japanese Unexamined Patent Publication No. Hei 02-263794
[Patent document 2] Japanese Unexamined Patent Publication No. Hei 05-170430
[Patent document 3] Japanese Unexamined Patent Publication No. Hei 06-305887
[Patent document 4] Japanese Unexamined Patent Publication No. Hei 09-309799
[Patent document 5] Japanese Unexamined Patent Publication No. 2000-86400
[Patent document 6] Japanese Unexamined Patent Publication No. Hei 11-349399